The receiver comes with a built-in linux kernel that downloads a new kernel (in the .arf) file. Is it possible to reflash the rio receiver to use a different kernel?

Not easily, no.

Define easily - it's possible with the serial adaptor, ISTR that if you have an intel flash chip in your box (not all do), the in-flash reflasher should work. It's the same (basically) as the empeg-car one, so download.c on the empeg website will work.

Hugo